Multi-range time of flight sensing

ABSTRACT

An electro-optical device includes at least one laser light source and a beam steering device, which transmits and scan the at least one beam across a target scene. One or more sensing elements output a signal indicative of a time of incidence of a single photon on the sensing element from the target scene. Circuitry processes the signal in order to determine respective distances to points in the scene and controls the light source to emit the beam at the low level during a first scan, to identify, based on the first scan, the points in the scene that are located at respective distances from the device that are greater than a predefined threshold distance, and to control the laser light source during a second scan to emit the beam at the high level while the beam steering device directs the beam toward the identified points.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional PatentApplication 62/353,589, filed Jun. 23, 2016.

FIELD OF THE INVENTION

The present invention relates generally to opto-electronic devices, andparticularly to light detection and ranging (LiDAR) sensors.

BACKGROUND

Existing and emerging consumer applications have created an increasingneed for real-time three-dimensional imagers. These imaging devices,also commonly known as light detection and ranging (LiDAR) sensors,enable the remote measurement of distance (and often intensity) of eachpoint on a target scene—so-called target scene depth—by illuminating thetarget scene with an optical beam and analyzing the reflected opticalsignal. A commonly used technique to determine the distance to eachpoint on the target scene involves sending an optical beam towards thetarget scene, followed by the measurement of the round-trip time, i.e.time-of-flight (ToF), taken by the optical beam as it travels from thesource to target scene and back to a detector adjacent to the source.

A suitable detector for ToF-based LiDAR is provided by a single-photonavalanche diode (SPAD) array. SPADs, also known as Geiger-mode avalanchephotodiodes (GAPDs), are detectors capable of capturing individualphotons with very high time-of-arrival resolution, of the order of a fewtens of picoseconds. They may be fabricated in dedicated semiconductorprocesses or in standard CMOS technologies. Arrays of SPAD sensors,fabricated on a single chip, have been used experimentally in 3D imagingcameras. Charbon et al. provide a useful review of SPAD technologies in“SPAD-Based Sensors,” published in TOF Range-Imaging Cameras(Springer-Verlag, 2013), which is incorporated herein by reference.

In a SPAD, a p-n junction is reverse-biased at a level well above thebreakdown voltage of the junction. At this bias, the electric field isso high that a single charge carrier injected into the depletion layer,due to an incident photon, can trigger a self-sustaining avalanche. Theleading edge of the avalanche current pulse marks the arrival time ofthe detected photon. The current continues until the avalanche isquenched by lowering the bias voltage down to or below the breakdownvoltage. This latter function is performed by a quenching circuit, whichmay simply comprise a high-resistance ballast load in series with theSPAD, or may alternatively comprise active circuit elements.

SUMMARY

Embodiments of the present invention that are described hereinbelowprovide improved LiDAR sensors and methods of their use.

There is therefore provided, in accordance with an embodiment of thepresent invention, an electro-optical device, which includes at leastone laser light source configured to emit at least one beam of lightpulses having an emissive power selectable between a low level and ahigh level. A beam steering device is configured to transmit and scanthe at least one beam across a target scene. One or more sensingelements are configured to output a signal indicative of a time ofincidence of a single photon on the sensing element. Light collectionoptics are configured to image the target scene scanned by the at leastone transmitted beam onto the one or more sensing elements.

Circuitry is coupled to process signals output by the one or moresensing elements in order to determine respective distances to points inthe target scene, to control the at least one laser light source to emitthe at least one beam at the low level during a first scan of the beamsteering device over the target scene, to identify, based on the firstscan, the points in the scene that are located at respective distancesfrom the device that are greater than a predefined threshold distance,and to control the at least one laser light source during a second scanof the beam steering device, subsequent to the first scan, to emit theat least one beam at the high level while the beam steering devicedirects the at least one beam toward the identified points.

In the disclosed embodiments, the at least one laser light sourceincludes a laser light source having an output selectable between thelow level and the high level. Alternatively or additionally, the atleast one laser light source includes at least two lasers, including atleast a first laser configured to emit the light pulses at the lowlevel, and at least a second laser configured to emit the light pulsesat the high level. In some embodiments, the circuitry is configured toset at least one of a timing and a sensitivity of the sensing elementsto different, respective values during the first scan and during thesecond scan.

In a disclosed embodiment, the circuitry is configured to control the atleast one laser light source during the second scan to direct the atleast one beam at the high level only at the points that were identifiedas being located at respective distances from the device that aregreater than the predefined threshold distance.

In some embodiments, the one or more sensing elements include an arrayof the sensing elements, and the circuitry is configured to actuate thesensing elements only in a selected region of the array and to sweep theselected region over the array in synchronization with scanning of theat least one beam. In a disclosed embodiment, the sensing elementsinclude single-photon detectors, such as single-photon avalanche diodes(SPADs).

There is also provided, in accordance with an embodiment of theinvention, a method for sensing, which includes selecting a laser lightsource emissive power between a low level and a high level, and emittingat least one beam of light pulses having a selected emissive power. Theat least one beam is transmitted and scanned across a target scene. Oneor more sensing elements are provided and are configured to output asignal indicative of a time of incidence of a single photon on thesensing element. The target scene scanned by the at least onetransmitted beam is imaged onto the one or more sensing elements.Signals output by the one or more sensing elements are processed inorder to determine respective distances to points in the target scene.The at least one laser light source is controlled to emit the at leastone beam at the low level during a first scan of the beam steeringdevice over the target scene. Based on the first scan, the points in thescene that are located at respective distances from the device that aregreater than a predefined threshold distance are identified, and the atleast one laser light source is controlled during a second scan of thebeam steering device, subsequent to the first scan, to emit the at leastone beam at the high level while directing the at least one beam towardthe identified points.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a LiDAR system, in accordance withan embodiment of the invention;

FIG. 2 is a block diagram that schematically illustrates a SPAD-basedsensing device, in accordance with an embodiment of the invention;

FIG. 3 is a block diagram showing components of a sensing element in aSPAD array, in accordance with an embodiment of the invention;

FIG. 4 is a block diagram that schematically illustrates a SPAD arraywith a scanned region of sensitivity, in accordance with an embodimentof the invention;

FIG. 5 is a schematic illustration of a detector array with a circularscanned illumination spot, in accordance with an embodiment of theinvention;

FIG. 6 is a schematic illustration of a detector array with a circularscanned illumination spot, in accordance with another embodiment of theinvention;

FIGS. 7A-C are a schematic illustrations of a detector array with anelliptical scanned illumination spot, in accordance with yet anotherembodiment of the invention;

FIG. 8 is a schematic illustration of a detector array with two circularillumination spots scanned in a two-dimensional raster scan, inaccordance with an embodiment of the invention;

FIG. 9 is a schematic illustration of a detector array with a staggeredarray of illumination spots scanned in a one-dimensional scan, inaccordance with an embodiment of the invention;

FIG. 10 is a schematic illustration of a LiDAR device implementing aone-dimensional scan, in accordance with an embodiment of the invention;

FIG. 11 is a schematic illustration of a LiDAR device implementing aone-dimensional scan, in accordance with another embodiment of theinvention;

FIG. 12 is a schematic illustration of a LiDAR device using a laserlight source with adjustable emissive power, in accordance with anembodiment of the invention; and

FIG. 13 is a schematic illustration of a LiDAR device using two laserlight sources with different emissive powers, in accordance with anembodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

The quality of the measurement of the distance to each point in a targetscene (target scene depth) using a LiDAR is often compromised inpractical implementations by a number of environmental, fundamental, andmanufacturing challenges. An example of environmental challenges is thepresence of uncorrelated background light, such as solar ambient light,in both indoor and outdoor applications, typically reaching anirradiance of 1000 W/m². Fundamental challenges are related to lossesincurred by optical signals upon reflection from the target scenesurfaces, especially due to low-reflectivity target scenes and limitedoptical collection aperture, as well as electronic and photon shotnoises. These limitations often generate inflexible trade-offrelationships that typically push the designer to resort to solutionsinvolving large optical apertures, high optical power, narrowfield-of-view (FoV), bulky mechanical construction, low frame rate, andthe restriction of sensors to operate in controlled environments.

The embodiments of the present invention that are described hereinaddress the above limitations so as to enable compact, low-cost LiDARsachieving accurate high-resolution depth imaging that can operate inuncontrolled environments. The disclosed embodiments use one or morepulsed laser sources emitting beams to generate high-irradianceillumination spots at the intersections of the axes of the emitted beamswith the target scene. The beams and hence the illumination spots arescanned across the target scene. The illumination reflected from thetarget scene is imaged by collection optics onto and detected by atime-of-flight, single-photon detector array for high signal-to-noiseratio, with the distance to each point of the target scene derived fromthe time-of-flight data.

Imaging of the target scene onto the detector array generates aone-to-one correspondence between locations in the target scene andlocations on the detector array, defined by geometrical optics, as isknown in the art. Thus, an area of the target scene is imaged onto acorresponding image area on the detector, with a linear length in theimage given by multiplying the corresponding length in the target scenearea by the optical magnification M, wherein for LiDAR systems typicallyM<<1. Similarly, a sensing element of the detector array can be thoughtof as imaged back onto the target scene with magnification 1/M, givingthe location and area of the target scene that is “seen” by the sensingelement.

In the disclosed embodiments, the detector array comprises atwo-dimensional array of single-photon time-sensitive sensing elements,such as single-photon avalanche diodes (SPADs). By addressing each SPADindividually via a dedicated control circuitry, the sensitivity,including the on/off-state, of each SPAD is controlled by its specificreverse p-n junction high voltage. In some embodiments the SPADs work asindividual sensing elements, whereas in other embodiments several SPADsare grouped together into super pixels. At any instant during a scan,only the sensing elements in the area or areas of the array that are toreceive reflected illumination from a scanned beam are actuated. Thesensing elements are thus actuated only when their signals provideuseful information. This approach both reduces the background signal,which would lower the signal-to-background ratio, and lowers theelectrical power needs of the detector array.

A LiDAR measures the distance to the target scene for a set of discretepoints with a finite averaging area associated with each point. In thedisclosed embodiments, the parameters of the measurement, as well as theactuation of sensing elements, are affected by the following systemparameters of the LiDAR:

1) The size of the illumination spots,

2) The resolution of the beam steering device (the size of the steps orexcursion of the beam steering device between successive measurements ofdistance), and

3) The size of the super pixels of the detector array, or in otherwords, the number of sensing elements that are binned together in theToF measurement (including the case in which one sensing element is usedas a super pixel).

The effect of the LiDAR system parameters can be divided into two cases:

a) Small-spot case, wherein the illumination spot is smaller than thesize of the super pixel, and

b) Large-spot case, wherein the illumination spot is larger than thesize of the super pixel

The comparison of sizes is made by viewing both the illumination spotand the super pixel in the same optical plane (either in the targetscene or on the detector array). These two cases can be summarized inthe following table, with the cases detailed further in the context ofthe figures.

TABLE 1 Impact of LiDAR system parameters a) Illumination spot < b)Illumination spot > Super pixel Super pixel Location of Intersectionpoint of Super pixel location measurement the beam axis with asprojected onto the point in the target scene target scene target scenedetermined by: Lateral Beam steering device Super pixel size asresolution resolution projected onto the in target target scene scenedetermined by: Measurement Illumination spot Super pixel size asaveraging size projected onto the area on target scene target scenedetermined by: State of Measurement point Selection of actuated beamlocation in target super pixels steering scene; device Selection ofactuated determines: super pixels

In some embodiments of the present invention, the target scene isilluminated and scanned by either one laser beam or by multiple beams.In some embodiments utilizing multiple beams, these beams are generatedby splitting a laser beam using diffractive optical elements, prisms,beamsplitters, or other optical elements that are known in the art. Inother embodiments, multiple beams are generated using several discretelaser light sources. In some of these embodiments, the multiple beamsare generated using a monolithic laser array, such as an array of VCSELsor VECSELs.

In some embodiments, a beam steering device, such as a scanning mirror,is operated to scan the target scene with a single light beam in atwo-dimensional raster scan. (A raster scan generally comprises long,approximately straight back-and-forth scans, so-called scan lines, alongwith short movements transferring the scan point from one scan line tothe next). A raster pattern is described here by way of example, andalternative scan patterns implementing similar principles are consideredto be within the scope of the present invention. When a single lightbeam is used, the scan resolution in the direction perpendicular to thescan lines of the raster scan is given by the separation betweensuccessive scan lines. The scan resolution can be increased bydecreasing the separation between successive scan lines, but this sortof resolution increase will come at the expense of reduced frame rate,since a larger number of scan lines is required to cover the scene.Alternatively, the resolution may be increased at the expense of reducedfield of view if the number of scan lines per frame is unchanged.Mechanical constraints put a limit on the degree to which the scanningspeed of the mirror can be increased in order to offset these effects.

In an embodiment, the scan resolution in the direction perpendicular tothe scan lines is increased by using multiple light beams, spreadtransversely relative to the scan line direction as well as in the scanline direction. The separation of the light beams along the scan line isconfigured so that each light beam illuminates a separate super pixel onthe detector array, in order to identify individually each light beam.The transverse separation of the light beams, rather than the scan linedensity, now determines the scan resolution. The disclosed embodimentachieves an increase in the lateral resolution without reducing the sizeof the sensing elements, thus mitigating the miniaturizationrequirements for the detector array.

In another embodiment, multiple illumination spots are scanned acrossthe target scene in a linear scan. (A linear scan in this contextincludes scans along a single direction in which the scan line isdistorted from a straight line due to optical or mechanicalimperfections.) Employing a one-dimensional, linear scan permits the useof a simpler and cheaper beam steering device than for a two-dimensionalscan, but the number of light beams to cover the target scene withsufficiently high resolution is generally higher than that required fora two-dimensional scan. A single-column scan can be implemented withmultiple light beams configured in a column perpendicular to the scanline, generating one column of illumination spots. The highest scanresolution in the direction of the axis of the column is attained wheneach illumination spot is imaged onto a separate sensing element in thedetector array.

In another embodiment utilizing a linear scan, the scan resolutionperpendicular to the scan line is increased by generating multiplecolumns of illumination spots, perpendicular to the scan line andmutually offset in the direction of the axes of the columns. Themultiple columns are also mutually offset in the direction of the scanline by at least one sensing element, so as to have each illuminationspot illuminate a separate sensing element, and thus permit eachillumination spot to be separately identified. This embodiment achievesan increase in the lateral resolution without reducing the size of thesensing elements, thus mitigating the miniaturization requirements forthe detector array.

Some embodiments of the present invention provide LiDAR systems with awide angular field-of-view (FoV), covering a large depth range. Asimplementing high-efficiency, wide-FoV optics leads to bulky andexpensive components, these embodiments apply dedicated designs anduse-modes of laser light sources, detector arrays, electronics, andalgorithms to measure scene depths over a wide range of FoVs anddistances, while keeping the optical design and construction simple.

The considerations for the laser light source relate to its emissivepower: If one were to use only low emission-power laser light sourcesfor target scene scanning, the signal received by the detector arrayfrom distant points of the target scene would be too weak for a robustand accurate measurement. If, on the other hand, one were to use onlyhigh emission-power laser light sources, capable of measuring distanttarget scene points, unnecessarily high emissive power would be used bythe LiDAR for nearby target scene points, increasing the electricalpower consumption of the LiDAR. Therefore, in some embodiments of theinvention, the laser light source emissive power is adjusted accordingto the measured distance.

System Description

FIG. 1 shows schematically a LiDAR system 18, in accordance with anembodiment of the invention. The beam or beams from a laser light source20, comprising one or more pulsed lasers, are directed to a target scene22 by a dual-axis beam-steering device 24, forming and scanningillumination spots 26 over the target scene. (The term “light” is usedherein to refer to any sort of optical radiation, including radiation inthe visible, infrared, and ultraviolet ranges.) Beam-steering devicescan comprise, for example, a scanning mirror, or any other suitable typeof optical deflector or scanner that is known in the art. Illuminationspots 26 are imaged by collection optics 27 onto a two-dimensionaldetector array 28, comprising single-photon, time-sensitive sensingelements, such as SPADs.

Target scene 22 is also illuminated, besides illumination spots 26, byan ambient light source 36, such as the sun. In order to achieve a highsignal-to-background ratio, the irradiance of the illumination spots ischosen to be much higher than that of the ambient illumination, whichcan reach up to 1000 W/m² due to irradiance from the sun, for example. Aband-pass filter 37 is used for further reduction of ambientillumination on detector array 28.

A control circuit 38 is connected to laser light source 20, timing thepulse emissions and controlling their emissive power, and to dual-axisbeam-steering device 24, controlling the scan of illumination spots 26.In addition, control circuit 38 adjusts dynamically the reverse p-njunction high voltage of each SPAD of detector array 28, thuscontrolling the actuation and sensitivity of each SPAD. Utilizing theknown timing of pulses from laser light source 20 and the known state ofdual-axis beam-steering device 24, which determines the position ofillumination spots 26 on target scene 22, control circuit actuates onlythose SPADs onto which, at any given moment, the illumination spots areimaged by collection optics 27. Utilizing further the above knowledge oflaser light source 20 and beam steering device 24, as well as thesignals read from detector array 28, control circuit 38 determines thedistance to each scanned point in target scene 22 using thetime-of-flight measured from the laser light source to the detectorarray.

FIGS. 2-4 illustrate schematically the architecture and functioning ofdetector array 28, in accordance with embodiments of the invention.These figures show one possible scheme that can be used for selectivelyactuating the SPAD-based sensing elements in the array, using acombination of global and local bias controls. Alternatively, othersorts of biasing and actuation schemes, as well as other sorts ofsingle-photon sensing elements, may be used for these purposes.

FIG. 2 is a block diagram that schematically illustrates detector array28, in accordance with an embodiment of the invention. Detector array 28comprises sensing elements 44, each comprising a SPAD and associatedbiasing and processing circuits, as described further hereinbelow. Aglobal high-voltage bias generator 46 applies a global bias voltage toall of sensing elements 44 in array 28. In addition, a local biasingcircuit 48 in each sensing element 44 applies an excess bias, which sumswith the global bias in the sensing element. A sensing element biascontrol circuit 50 sets the excess bias voltages applied by localbiasing circuits 48 to different, respective values in different sensingelements. Both global high-voltage bias generator 46 and sensing elementbias control circuit 50 are connected to control circuit 38 (FIG. 1).

FIG. 3 is a block diagram showing components of one of sensing elements44 in array 28, in accordance with an embodiment of the invention. Inthe disclosed embodiment array 28 comprises a two-dimensional matrix ofthe sensing elements formed on a first semiconductor chip 52, with asecond two-dimensional matrix of bias control and processing circuitsformed on a second semiconductor chip 54. (Only a single element of eachof the two matrices is shown.) Chips 52 and 54 are coupled together sothat the two matrices are in a one-to-one correspondence, whereby eachsensing element on chip 52 is in contact with the corresponding biascontrol and processing elements on chip 54.

Both of chips 52 and 54 may be produced from silicon wafers usingwell-known CMOS fabrication processes, based on SPAD sensor designs thatare known in the art, along with accompanying bias control andprocessing circuits as described herein. Alternatively, the designs andprinciples of detection that are described herein may be implemented,mutatis mutandis, using other materials and processes. For example, allof the components shown in FIG. 3 may be formed on a single chip, or thedistribution of the components between the chips may be different. Allsuch alternative implementations are considered to be within the scopeof the present invention.

Sensing element 44 comprises a SPAD 56, comprising a photosensitive p-njunction, as is known in the art. Peripheral circuits including aquenching circuit 58 and local biasing circuit 48, are typically locatedon chip 54. As explained above, the actual bias applied to SPAD 56 is asum of the global bias voltage V_(bias) provided by bias generator 46(FIG. 2) and an excess bias applied by biasing circuit 48. Sensingelement bias control circuit 50 (FIG. 2) sets the excess bias to beapplied in each sensing element by setting a corresponding digital valuein a bias memory 60 on chip 54.

In response to each captured photon, SPAD 56 outputs an avalanche pulse,which is received by processing circuits on chip 54, including digitallogic 62 and a memory configured as an output buffer 64. Theseprocessing elements can be configured, for example, to function as atime-to-digital converter (TDC), which measures the delay of each pulseoutput by SPAD 56 relative to a reference time and outputs a digitaldata value corresponding to the delay. Alternatively or additionally,logic 62 and buffer 64 may measure and output other sorts of values,including (but not limited to) a histogram of pulse delay times, abinary waveform, or a multi-level digital waveform. The outputs fromchip 54 are connected to control circuit 38 (FIG. 1).

FIG. 4 is a block diagram that schematically illustrates SPAD array 28with a scanned region 70 of sensitivity, in accordance with anembodiment of the invention. In this case, bias control circuit 50 setsthe bias voltages of sensing elements 72 within region 70 to highervalues than the remaining sensing elements 76, wherein the bias voltageis set so that sensing elements 76 are turned off. Bias control circuit50 modifies the bias voltages of sensing elements 48 dynamically,however, so as to sweep region 70 across the array, as illustrated bythe arrow in the figure. Circuit 50 may, for example, sweep region 70 ina raster scan, in synchronization with the scanning of a laser beamacross a target scene being imaged onto array 28 (as is illustrated inthe figures that follow).

As noted earlier, this embodiment is useful, inter alia, in tailoringthe sensitive region of array 28 to the shape of an illuminating lightbeam or of an area of interest in a target scene being imaged, thusmaximizing sensitivity of array 28 relative to power consumption, whilereducing background noise from sensing elements that will not contributeto the signal.

In an alternative embodiment of the invention (illustrated in FIG. 9,for example), bias control circuit 50 sets the local bias voltages sothat region 70 has a linear shape, extending along one or more columnsof array 28 and matching the linear shape of an illumination beam orarray of beams. Circuit 50 may then sweep this linear region 70 acrossarray 28 in synchronization with the illumination beam. Alternatively,other scan patterns may be implemented, including both regular andadaptive scan patterns.

Example Scan Patterns and Super Pixels

FIG. 5 is a schematic illustration showing detector array 28 with theimage of a circular scanned illumination spot 26 (FIG. 1) superimposedon the array, in accordance with an embodiment of the invention. Themoving image of illumination spot 26, as cast on detector array 28 bycollection optics 27, is observed at three consecutive points in time:t=t_(i−1), t=t_(i), and t=t_(i+1). The images of scanned illuminationspot 26 for these three consecutive points in time are denoted bycircles 84, 86, and 88, respectively, with their diameters, in thisexample, twice the pitch of sensing elements 44. An arrow 90 indicatesthe direction of the scan of the image of scanned illumination spot 26,with the expected position of the image of the scanned illumination spotdetermined from the knowledge of the state of beam steering device 24.

At each point in time, sensing elements 44 in a region of array 28 thatbest matches the position of the image of illumination spot 26 at thatpoint of time are actuated. These actuated sensing elements can beregarded as a sort of “super pixel.” In the embodiment shown in FIG. 5,each super pixel comprises an array of 2×2 sensing elements, but in someembodiments the size of the super pixel takes other values eitherstatically or dynamically.

At the time t=t_(i−1) a super pixel 92 is actuated (encompassing circle84); at the time t=t_(i) a super pixel 94 is actuated (circle 86); andat the time t=t_(i+1) a super pixel 96 is actuated (circle 88). Thus, inthe pictured embodiment, each sensing element 44 is associated with twoneighboring super pixels. Only those sensing elements within the activesuper pixel are actuated at a given moment, with the rest of the sensingelements turned off by lowering their bias voltage to a level at whichavalanche multiplication is not sustainable. This operation maximizesthe collection of the optical signal from the image of scannedillumination spot 26, while reducing the exposure to target scenebackground illumination uncorrelated to the illumination spot, thusincreasing the signal-to-background ratio of array 28. In someembodiments of the invention, the output of the sensing elements thatare not illuminated by the image of scanning spot 26 are masked outusing standard logic gates.

The lateral resolution of target scene 22 in the direction of the scanis determined by the discrete step size of the scan (as determined bythe scan speed and laser pulse repetition rate), which in thisembodiment is one pitch of sensing elements 44. The area over which thetarget scene distance is averaged is (approximately) the area of a superpixel.

FIG. 6 is a schematic illustration showing detector array 28 with theimage of a circular scanned illumination spot 26 (FIG. 1) superimposedon the array, in accordance with another embodiment of the invention.The moving image of the illumination spot is observed at threeconsecutive points in time: t=t_(i−1), t=t_(i), and t=t_(i+1). Both thediameter of the image of the scanned illumination spot and the scanningstep between two consecutive points in time are half of the pitch ofsensing elements 44. The images of scanned illumination spot 26 for thethree consecutive points in time are denoted by circles 100, 102, and104, respectively. An arrow 105 indicates the direction of the scan,with the expected position of the image determined from the knowledge ofthe state of beam steering device 24. In this embodiment a super pixelof a single sensing element 44 is used, with super pixel 106 actuatedfor t=t_(i−1), and super pixel 108 actuated for both t=t_(i) and t=t₁₊₁.The lateral resolution of the target scene 22 image in the direction ofthe scan is half of the pitch of sensing elements 44, and the area oftarget scene over which the distance is averaged is the area ofillumination spot 26.

FIGS. 7A-C are schematic illustrations showing detector array 28 withthe image of an elliptical scanned illumination spot 26 (FIG. 1)superimposed on the array, in accordance with yet another embodiment ofthe invention. An elliptical illumination spot is obtained, forinstance, from an edge-emitting laser diode in which the emittingjunction cross-section is a rectangle with a high aspect ratio. In thisembodiment, an elliptical illumination spot 26 with an aspect ratio of3-to-1 is illustrated, although other aspect ratios can be used in otherembodiments. The extent of the so-called fast axis (long dimension) ofthe elliptical image of illumination spot 26 on detector array 28 isroughly six times the pitch of sensing elements 44, and the extent ofthe slow axis (short dimension) is twice the pitch. FIGS. 7A-C showschematically, similarly to FIGS. 5-6, the moving image of illuminationspot 26 at three consecutive points in time: t=t_(i−1), t=t_(i) andt=t_(i+1). Each scan step on detector array 28 is one pitch of sensingelements 44. In this embodiment, super pixels of 2×2 sensing elementsare used.

FIG. 7A shows schematically an illumination spot 110, which is the imageof scanned illumination spot 26 at the time t=t_(i−1). The super pixelsactuated at this time, based on the expected position of illuminationspot 110, are pixels 112, 114, and 116 (the farthest top and bottom tipsof the illumination spot are ignored, as they would contribute verylittle to the signal). An arrow 118 indicates the direction of the scan,with the expected position of illumination spot 110 determined from theknowledge of the state of beam steering device 24.

FIG. 7B shows schematically an illumination spot 120, which is the imageof scanned illumination spot 26 at the time t=t_(i). The super pixelsactuated at this time, based on the expected position of illuminationspot 120, are 112, 114, 116, and 122. Now four super pixels areactuated, since a significant portion of illumination spot 120 (top ofellipse) is still within pixel 112, and another significant portion(bottom of ellipse) has entered pixel 122. Super pixels 112, 114, and116 continue collecting the signal so as to improve the signal-to-noiseratio. As in FIG. 7A, arrow 118 indicates the direction of the scan,with the expected position of illumination spot 120 at t=t_(i)determined from the knowledge of the state of beam steering device 24.

FIG. 7C shows schematically an illumination spot 124, which is the imageof scanned illumination spot 26 at time t=t_(i+1). The super pixelsactuated at this time, based on the expected position of illuminationspot 124, are now 114, 116, and 122. Now only three super pixels areactuated, as pixel 112 (FIG. 7B) is no longer illuminated by anysignificant portion of illumination spot 124. As in FIGS. 7A-B, arrow118 indicates the direction of the scan, with the expected position ofillumination spot 124 at t=t₁₊₁ determined from the knowledge of thestate of beam steering device 24. In the pictured embodiment, each superpixel will be exposed to the image of illumination spot 26 for sevenscan steps, thus improving the signal-to-noise ratio.

Due to the fact that the length of the elliptical illumination spot ismuch larger than the super pixel, the resolution in the direction of thescan is determined by the super pixel size. As the super pixel size is athird of the length of the elliptical illumination spot along its fast(long) axis, the resolution obtained in the direction of the scan lineis three times as good (a third in numerical value) as that obtainedwith the elliptical illumination spot alone. The averaging area for thedistance measurement is the area of a super pixel.

In FIGS. 5-7, an ideal shape (circular or elliptical) has been used asthe shape of the image of illumination spot 26 on detector array 28. Inan embodiment of the present invention, control circuit 38 calculates(or looks up) the actual shape of the illumination spot image on thedetector array, and the results of this calculation are used in choosingthe sensor elements to be activated at each point in the scan. Thecalculation takes into account the effects of the design of beamsteering device 24, its scanning movement characteristics, the exactstate of the beam steering device, and the angle between the beam fromlaser light source 20 and the beam steering device, as they impact onthe shape, direction of movement, and orientation of the image ofillumination spot 26. In addition, the dependence of the image on thedistance between the LiDAR device and target scene 22 is taken intoaccount. This effect is significant, especially for target scene rangesthat are short as compared to the separation distance between beamsteering device 24 and collection optics 27. The above calculations areperformed in order to obtain the best overlap between actuated sensingelements 44 and illumination spot 26 image on detector array 28, whileachieving the desired vertical and horizontal angular resolution, thusoptimizing the signal-to-background and signal-to-noise figures.

FIG. 8 is a schematic illustration showing a technique for enhancementof the resolution of a raster-scanning LiDAR, in accordance with anembodiment of the invention. Beam steering device 24 scans the images ofillumination spots 26 (FIG. 1) on detector array 28 in a raster scanpattern 130 down one column and up the next column of the detectorarray. If only one illumination spot were to be used, the lateralresolution perpendicular to the scan lines of the raster scan would bethe pitch of sensing elements 44. In the present embodiment, however,the lateral resolution is doubled by using two scanned illuminationspots 26, whose images on detector array 28 are separated along the scanline by a distance equal to the pitch of sensing elements 44, andtransversely to the scan line by half of this pitch. Beam steeringdevice 24 and the repetition rate of laser light source 20 areconfigured so that successive illumination spots are separated by stepsof half the pitch of sensing elements 44 in the direction of the scanline of the raster scan. Each super pixel comprises one sensing element44.

FIG. 8 shows schematically the images of two illumination spots 26 attwo consecutive points in time: t=t_(i) and t=t_(i+1). At the timet=t_(i), the images of the illumination spots are a spot 132 and a spot134, with spot 132 inside a super pixel 136, and spot 134 inside a superpixel 138. All other super pixels are turned off. At the time t=t_(i+1),both spots have moved down, as indicated by arrows 140, by half a superpixel, to new positions 142 and 144. The spots are still inside the samesuper pixels 136 and 138 as at t=t_(i), but the positions ofillumination spots 142 and 144 are determined by the state of beamsteering device 24 at the time t=t_(i+1). Due to the fact that the twospots are at all times assigned to separate super pixels, the spots areindividually identifiable, and the resolution of the LiDAR transverselyto the scan line is determined by the separation of the images of thetwo illumination spots 26 in that direction, rather than the pitch ofsensing elements 44, thus mitigating the miniaturization requirementsfor detector array 28. The averaging area of distance measured by eachof illumination spots 26 is the area of that illumination spot.

In another embodiment (not shown in the figures) the number of scannedillumination spots 26 is increased to more than two (as compared to FIG.8), with the illumination spots separated along raster scan pattern 130so that the image of each illumination spot is located in a differentsensing element 44. For an embodiment in which the images of Nillumination spots 26 are all within one column of detector array 28,the resolution transversely to raster scan 130 is given by dividing thepitch of sensing elements 44 by N.

Linear Scan Patterns

FIGS. 9-11 are schematic illustrations showing a LiDAR based on a linearscan, in accordance with an embodiment of the invention. A linear(one-dimensional) scan has the advantage that it utilizes a potentiallysmaller, cheaper, and more reliable design of the beam steering devicethan that required for a two-dimensional scan. The resolution in thedirection of the linear scan is determined by the resolution of the beamsteering device. As no scan takes place transversely to the direction ofthe linear scan, resolution in that direction is accomplished by usingmultiple illumination spots 26 arrayed across target scene 22.

FIG. 9 is a schematic illustration showing a one-dimensional scan asimaged onto detector array 28, in accordance with an embodiment of theinvention. The resolution of the LiDAR in the direction perpendicular tothe linear scan is improved beyond the pitch of sensing elements 44 byusing a pattern 150 of images of illumination spots 26 comprising twostaggered columns 151 and 152, with circles 153 denoting the expectedpositions of the images of the individual illumination spots on sensorarray 28. Arrows 154 indicate the direction of the scan.

In each column 151 and 152 of pattern 150, the spacing of the images ofillumination spots 26, as indicated by circles 153, along the axis ofthe respective column is equal to the pitch of sensing elements 44. Thetwo columns 151 and 152 are mutually offset by half of the pitch ofsensing elements 44 in the direction of the axes of the columns. Columns151 and 152 are spaced in the direction of the scan by one pitch inorder to assign the two columns to separate sensing elements. In someembodiments (not shown in figure), the resolution transverse to thelinear scan is further improved by using more than two columns ofillumination spots 26 with smaller mutual offsets in the direction ofthe axes of the columns. Thus, for example, using four columns withmutual offsets of a quarter pitch of sensing elements 44, a resolutionof one quarter pitch is achieved.

FIG. 10 is a schematic illustration showing a LiDAR 159 based on aone-dimensional scan, in accordance with an embodiment of the invention.The beam from a single pulsed laser source 160 is split by a diffractiveoptical element (DOE) 162 into two staggered columns of multiple beams.These beams are directed to and scanned over target scene 22 by asingle-axis beam-steering device 166, forming two staggered columns ofillumination spots 168 on target scene 22. The illumination spots areimaged by collection optics 27 onto detector array 28, forming twostaggered columns 151 and 152 in pattern 150 as shown in FIG. 9.

Only sensing elements 44 that contain images of illumination spots 26 inpattern 150 are actuated at any given instant during the scan, with theremaining sensing elements turned off, thus preventing unnecessaryintegration of background light, and achieving a highsignal-to-background ratio. Similarly to FIG. 1, control circuit 38 isconnected to laser light source 160, beam steering device 166, anddetector array 28, controlling their functions and collecting data todetermine the distance to target scene 22 by using time-of-flight data.

FIG. 11 is a schematic illustration showing a LiDAR 170 based on aone-dimensional scan and a co-axial optical architecture, in accordancewith another embodiment of the invention. The beam from a single pulsedlaser source 160 is split by DOE 162 into two staggered columns ofmultiple beams. These beams pass through a polarizing beamsplitter 176,and are directed to and scanned over target scene 22 by single-axisbeam-steering device 166, thus forming two staggered columns ofillumination spots 168. The illumination spots, reflected from targetscene 22, are imaged through beam steering device 166, polarizingbeamsplitter 176, and collection optics 27 onto detector array 28,forming two staggered columns 151 and 152 in pattern 150 as shown inFIG. 9.

Due to the co-axial architecture of optical transmission and collection,pattern 150 on detector array is (nearly) stationary with respect to thescan. Therefore, the number of columns of sensor elements 44 on thedetector array along the axis perpendicular to the direction of the scanmay be considerably smaller than the number of rows of sensor elementsalong the scan direction. Similarly to FIG. 1, control circuit 38 isconnected to laser light source 160, beam steering device 166, anddetector array 28, controlling their functions and collecting data todetermine the distance to target scene 22 using time-of-flight data.

In both of the embodiments that are shown in FIGS. 10 and 11, thelateral resolution perpendicular to the scan direction is half of thepitch of sensing elements 44, and the resolution along the scan isdetermined by the scan rate of beam steering device 166 and the pulserepetition rate of laser source 160. Each one of illumination spots 168averages the distance measurement over the area of that spot.

The vertical orientation of columns 151 and 152 in pattern 150 is shownhere by way of example, and alternative orientations implementingsimilar principles are considered to be within the scope of the presentinvention.

Multi-Range Sensing

FIGS. 12-13 are schematic illustrations showing LiDARs that adaptthemselves to near and far distances of the target scene, in accordanceto embodiments of the invention.

FIG. 12 is a schematic illustration showing a LiDAR 199, which adaptsitself for measuring distances to both near and far target scene points,in accordance with an embodiment of the invention. The beam of a pulsedlaser light source 200 is directed to target scene 22 by dual-axisbeam-steering device 24, forming an illumination spot 206 on andscanning the spot over the target scene. Illumination spot 206 is imagedonto detector array 28 by collection optics 27. Control circuit 38 isconnected to laser light source 200, beam steering device 24, anddetector array 28.

Laser light source 200 has the capability to emit light at two powerlevels: low emissive power and high emissive power, under control ofsignals from control circuit 38. Concomitantly, sensing elements 44 ofdetector array 28 (see FIG. 2) have the capability to operate in twodistinct modes: short-range mode and long-range mode. For a given modeof operation of a specific sensing element, control circuit 38 willadjust its timing and sensitivity, as well as the signal processingalgorithms for optimal performance in that mode. Typically, inshort-range mode, sensing elements 44 are biased for relatively lowersensitivity (which also results in lower noise) and gated to sense shorttimes of flight. In the long-range mode, sensing elements 44 are biasedfor relatively higher sensitivity and gated to sense longer times offlight, thus reducing the likelihood of spurious detection ofshort-range reflections.

To determine the required mode of operation for each area of targetscene 22, the area is first scanned using laser light source 200 at itslow emissive power level, suitable for short-range detection. Thesensing elements 44 in detector array 28 receiving the light originatingfrom laser light source 200 are actuated with their timing, sensitivity,and associated signal processing algorithms set for short-range distancemeasurement.

Following this short-range scan, control circuitry 38 controls LiDAR 199to perform a long-range scan only in the areas in which, based onpredetermined criteria, the short-range, low-power scan did not yield asufficiently robust distance measurement. In the long-range scan, themeasurement for these areas is repeated using the high emissive powerlevel of light source 200, with appropriate changes in the timing,sensitivity, and algorithms of sensing elements 44 that are actuated toreceive the reflected light from these areas.

FIG. 13 is a schematic illustration showing a LiDAR 210, which adaptsitself for measuring distances to both near and far target scene points,in accordance with another embodiment of the invention. The beams of twopulsed laser light sources 218 and 220 are directed to target scene 22by dual-axis beam-steering device 24, forming an illumination spot 226on and scanning it over target scene 22. (The separation between laserlight sources 218 and 220 is exaggerated in FIG. 13 in order to show thetwo separate sources). As will be detailed below, only one of the laserlight sources is emitting at one given time. Illumination spot 226 isimaged onto detector array 28 by collection optics 27. Control circuit38 is connected to laser light sources 218 and 220, beam steering device24, and detector array 28.

Each laser light source 218, 220, when actuated, emits at a specificemissive power level, with laser light source 218 emitting at a lowemissive power level, and laser light source 220 emitting at a highemissive power level. Control circuit 38 chooses which of the laserlight sources to actuate at each point in the scan based on the sorts ofcriteria explained above with reference to FIG. 12. Similarly, sensingelements 44 of detector array 28 (see FIG. 2) have the capability tooperate in two distinct modes: short-range mode and long-range mode. Fora given mode of operation of a specific sensing element 44, controlcircuit 38 will adjust its timing and sensitivity as well as its signalprocessing algorithms for optimal performance in that mode.

To determine the required mode of operation in a given area of targetscene 22, the area is first scanned using low emissive power laser lightsource 218. Those sensing elements 44 in detector array 28 that receivethe light originating from laser light source 218 are actuated, withtheir timing, sensitivity, and associated signal processing algorithmsset for short-range distance measurement. As in the precedingembodiment, if control circuit 38 determines that a sufficiently robustdistance measurement cannot be made for a given area using laser lightsource 218, the measurement for that area is repeated at higher emissivepower laser using light source 220, with appropriate changes in thetiming, sensitivity, and algorithms of those of sensing elements 44 thatare actuated for receiving the light from laser light source 220.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

1. An electro-optical device, comprising: at least one laser lightsource configured to emit at least one beam of light pulses having anemissive power selectable between a low level and a high level; a beamsteering device configured to transmit and scan the at least one beamacross a target scene; one or more sensing elements configured to outputa signal indicative of a time of incidence of a single photon on thesensing element; light collection optics configured to image the targetscene scanned by the at least one transmitted beam onto the one or moresensing elements; and circuitry coupled to process signals output by theone or more sensing elements in order to determine respective distancesto points in the target scene, to control the at least one laser lightsource to emit the at least one beam at the low level during a firstscan of the beam steering device over the target scene, to identify,based on the first scan, the points in the scene that are located atrespective distances from the device that are greater than a predefinedthreshold distance, and to control the at least one laser light sourceduring a second scan of the beam steering device, subsequent to thefirst scan, to emit the at least one beam at the high level while thebeam steering device directs the at least one beam toward the identifiedpoints.
 2. The device according to claim 1, wherein the at least onelaser light source comprises a laser light source having an outputselectable between the low level and the high level.
 3. The deviceaccording to claim 1, wherein the at least one laser light sourcecomprises at least two lasers, comprising at least a first laserconfigured to emit the light pulses at the low level, and at least asecond laser configured to emit the light pulses at the high level. 4.The device according to claim 1, wherein the circuitry is configured toset at least one of a timing and a sensitivity of the sensing elementsto different, respective values during the first scan and during thesecond scan.
 5. The device according to claim 1, wherein the circuitryis configured to control the at least one laser light source during thesecond scan to direct the at least one beam at the high level only atthe points that were identified as being located at respective distancesfrom the device that are greater than the predefined threshold distance.6. The device according to claim 1, wherein the one or more sensingelements comprise an array of the sensing elements, and wherein thecircuitry is configured to actuate the sensing elements only in aselected region of the array and to sweep the selected region over thearray in synchronization with scanning of the at least one beam.
 7. Thedevice according to claim 6, wherein the circuitry is configured toselect the region such that at any instant during the scan, the selectedregion contains a part of the array onto which the light collectionoptics image an area of the target scene that is illuminated by the atleast one beam.
 8. The device according to claim 6, wherein the at leastone laser light source is configured to emit at least two beams alongdifferent, respective beam axes, such that at any instant during thescan, the light collection optics image respective areas of the targetscene that are illuminated by the at least two beams onto different,respective ones of the sensing elements.
 9. The device according toclaim 1, wherein the sensing elements comprise single-photon detectors.10. The device according to claim 9, wherein the single-photon detectorsare single-photon avalanche diodes (SPADs).
 11. A method for sensing,comprising: selecting a laser light source emissive power between a lowlevel and a high level; emitting at least one beam of light pulseshaving a selected emissive power; transmitting and scanning the at leastone beam across a target scene; providing one or more sensing elementsconfigured to output a signal indicative of a time of incidence of asingle photon on the sensing element; imaging the target scene scannedby the at least one transmitted beam onto the one or more sensingelements; controlling the at least one laser light source to emit the atleast one beam at the low level during a first scan of the beam steeringdevice over the target scene; processing signals output by the one ormore sensing elements in order to determine respective distances topoints in the target scene; identifying, based on the first scan, thepoints in the scene that are located at respective distances from thedevice that are greater than a predefined threshold distance; andcontrolling the at least one laser light source during a second scan ofthe beam steering device, subsequent to the first scan, to emit the atleast one beam at the high level while directing the at least one beamtoward the identified points.
 12. The method according to claim 11,wherein controlling the at least one laser light source switching anoutput level of a laser light source between the low level and the highlevel.
 13. The method according to claim 11, wherein controlling the atleast one laser light source comprises alternately selecting the outputof a first laser light source configured to emit the light pulses at thelow level, and selecting the output of a second laser light sourceconfigured to emit the light pulses at the high level.
 14. The methodaccording to claim 11, and comprising setting at least one of a timingand a sensitivity of the sensing elements to different, respectivevalues during the first scan and during the second scan.
 15. The methodaccording to claim 11, wherein controlling the at least one laser lightsource comprises setting the at least one laser light source during thesecond scan to direct the at least one beam at the high level only atthe points that were identified as being located at respective distancesfrom the device that are greater than the predefined threshold distance.16. The method according to claim 11, wherein the one or more sensingelements comprise an array of the sensing elements, and wherein themethod comprises actuating the sensing elements only in a selectedregion of the array and to sweep the selected region over the array insynchronization with scanning of the at least one beam.
 17. The methodaccording to claim 16, wherein actuating the sensing elements comprisesselecting the region such that at any instant during the scan, theselected region contains a part of the array onto which the lightcollection optics image an area of the target scene that is illuminatedby the at least one beam.
 18. The method according to claim 16, whereinemitting at least one beam comprises emitting at least two beams alongdifferent, respective beam axes, such that at any instant during thescan, the light collection optics image respective areas of the targetscene that are illuminated by the at least two beams onto different,respective ones of the sensing elements.
 19. The method according toclaim 11, wherein the sensing elements comprise single-photon detectors.20. The method according to claim 15, wherein the single-photondetectors are single-photon avalanche diodes (SPADs).